A slight extension of the method has led to interesting and im portant results in connection with nebulae.
A comparison by Hale of ordinary photographs of the Orion nebula with exposures on panchromatic plates through a red filter shows conspicuous differences in colour in different parts of the nebula which indicate a variation in the mixture and radiation of its gaseous constituents. Again, similar photographs of spiral nebulae by Seares show that the central nuclei of these objects are yellow, in agreement with spectroscopic results, but that the knots and condensations on the spiral arms are blue. These results indicate a characteristic difference in physical conditions in differ ent parts of these nebulae.
Photography through coloured screens or filters of various kinds has been applied recently to the planets. Photographs of Mars and of Jupiter, by Wright of the Lick Observatory, taken with rays of the infra-red portion of the spectrum, with yellowy light, and with ultra-violet rays, show distinct differences, consideration of which appears to lead to knowledge of peculiarities of the surfaces and atmospheres of these bodies. Similar photographs of Venus by Ross give important information bearing on the physical char acteristics of this planet.
Colour photographs of the kind described represent a crude kind of spectrum analysis, in which attention is directed to the total radiation over a considerable range in wave-length rather than to the number, position and intensity of the spectral lines. The method is important and useful for objects too faint for detailed spectroscopic examination.
The astronomical applications of photography would be seriously restricted were there no means of extending the normal blue violet sensitivity of the ordinary photographic emulsion.
Vogel in 1873 discovered that various dye substances, notably naphtalinrot, conferred sensitiveness to other spectral regions. His results were confirmed and extended by Becquerel; by Waterhouse, who discovered the efficiency of eosin ; by Eder, who introduced erythrosin ; and by many others. To Vogel is due the discovery of the effectiveness of cyanin as a red sensitizer, and Miethe, E. Konig and Homolka, 1898-1905, made extensive studies of a great number of others, which led shortly to the production of panchromatic plates. Dicyanin and, more recently, kryptocyanin and neocyanin have played a great part in extending the sensitivity far into the infra-red. All modern research in colour sensitiza tion has been concerned with the study of dye substances. Abney's special red-sensitive emulsion seems never to have been repro duced.
The first great astronomical achievement resulting from the preparation of colour-sensitive plates was Abney's map of the in fra-red region of the solar spectrum to which reference has already been made; the next, Rowland's photographic map of the solar spectrum and the Table of Wave-Lengths of Spectrum Lines, ex tending from 2975A to 7331A, from measurement of photographs.
Rowland himself made the photographic plates that he used; the method of colour sensitization he employed is unknown. A recent application of colour photography to astronomy led to Hale's discovery of magnetic fields in sun-spots. Using Wallace's 3-dye sensitizer consisting of pinacyanol, pinaverdol, and homocol, Hale photographed with the spectroheliograph the distribution of hydro gen (Ha) over the solar disc. The presence of vortex structure centered about sun-spots suggested the possible existence of mag netic fields, which special observations soon fully confirmed.
Spectroscopic observation itself is now almost entirely photo graphic. In 1863 Huggins obtained photographic images of the spectra of Sirius and Capella but no clearly defined lines were shown, and attempts of this kind were suspended for some years. It is said that the first photograph showing spectral lines of a star was taken by Henry Draper in 1872, and in the years imme diately succeeding both these astronomers were successful in this way with the brightest stars. In 1882 each of them obtained a sat isfactory photograph of the spectrum of the Great Nebula in Orion. In 1886 a new spectrographic method was initiated at the Harvard College Observatory in connection with a scheme known as the Draper Memorial, a prism being placed before the object glass by which the spectra of all the stars in the field are formed and photographed simultaneously on the plate. The instrument then used was a photographic lens of 8 in. aperture with a prism 8 in. square fastened before it. The refracting angle of the prism was 13°, and the defracting edge was placed parallel to the equa tor, so that breadth could be given to the spectrum by altering the rate of the driving clock. Stars to about the 7th magnitude were thus shown. In 1887 Vogel introduced the photographic method at the Astrophysical Observatory at Potsdam for the determination of stellar radial velocity, and from that time, which is generally said to mark the beginning of accurate work of the kind all important spectroscopic work has been done in this way. Improvements and refinements in design of the spectrograph, especially the introduction of a temperature control for use during observations, were made by Campbell at the Lick Ob servatory. These afforded a new standard of precision in spectroscopic measurements, and led to an extended programme involving the systematic observation, at Mt. Hamilton and at Santiago, of 6,182 stars mostly brighter than visual magnitude 5.51. The resulting catalogue of radial velocities, issued in 1928, could not have been prepared without the aid of photography. Photographic methods have proved themselves equally indis pensable in every other field of spectroscopic observation. The measurement of the distance of a star from observations of its spectrum, by a method discovered by Adams and Kohlschtitter and developed by Adams at Mount Wilson, essentially requires the determination of the relative intensity of certain pairs of spectral lines. Star light, in general, is far below the limiting intensity which would permit even the crudest visual observation of these intensities, and this is often true of laboratory sources of light which must be studied as a means of interpreting the observations of celestial bodies. Even for the sun, which gives an abundance of light, measures are most advantageously made on photographs. In the infra-red and ultra-violet regions, outside the limits of visual perception, photographic methods alone are pos sible. The novel instrument known as the spectroheliograph con ceived independently by Prof. Hale and M. Deslandres, director of the National Observatory, Paris, is an ingenious adjunct to spectral photography. On a plate moving behind a second slit at the rear of a spectroscope at the same rate as the image crosses the primary slit, or by an equivalent device the sun is photographed by the light of a particular wave-length and pictures result which show the calcium or the hydrogen alone, in the successive envelopes of the sun, and so add vastly to our knowledge of its structure. The help that photography brings to eclipse observation by making a complete record of the spectrum of the chromosphere or of the corona in the very short time available on such occasions, that may be studied at leisure, need not be dwelt on.